Catalyst system for the treatment of exhaust gases from diesel engines

ABSTRACT

A catalyst system for the treatment of exhaust gases from a diesel engine includes a first and a second catalyst reducing catalyst arranged in series in an exhaust gas treatment system. The first catalyst is located near the engine in a region of the exhaust gas treatment system in which the exhaust gas temperature reaches temperatures of more than 200° C. under full engine load. The second catalyst is located further from the engine in a region of the exhaust gas treatment system in which the exhaust gas temperature reaches a maximum of 500° C. under full engine load. The maximum nitrogen oxides reduction in the first catalyst takes place at a lower temperature than the maximum nitrogen oxides reduction in the second catalyst.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a catalyst system for the treatment ofexhaust gases from diesel engines.

2. Description of the Related Art

The exhaust gas from diesel engines contains, during normal operatingphases, a high proportion, about 3 to 10 vol. %, of oxygen in additionto unburnt hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides(NO_(x)). Since the concentration of oxygen in the exhaust gas isgreater than that required stoichiometrically, it is not possible toconvert all three hazardous substances by the three-way processconventionally used with petrol engines. Petrol engines usually operatewith normalised air/fuel ratios λ close to 1, while diesel enginesoperate with normalised air/fuel ratios greater than 1.2. The normalisedair/fuel ratio λ is the air:fuel ratio (kilograms of air to kilograms offuel) standardized to stoichiometric operation.

The composition of the exhaust gas from diesel engines is very dependenton the particular operating phase of the engine. In the cold-startphase, that is the first 60 to 120 seconds after starting the engine,the exhaust gas has a high concentration of hydrocarbons, but theconcentration of nitrogen oxides is still low. With longer operatingtimes and under higher engine loads, the emissions of unburnthydrocarbons decrease and emissions of nitrogen oxides increase.

The unburnt hydrocarbons and carbon monoxide in diesel gas exhausts canbe converted relatively easily by oxidizing catalysts.

This type of catalyst is described, for example, in DE 39 40 758 C2.This comprises an oxidizing catalyst with high rates of conversion forhydrocarbons and carbon monoxide and an inhibited oxidizing effecttowards nitrogen oxides and sulfur dioxide. Nitrogen oxides can passover the catalyst virtually unchanged.

Nitrogen oxides can be converted only by using special reducingcatalysts, due to the high oxygen content of diesel exhaust gas.Basically this means that these reducing catalysts also exhibit a highoxidizing effect towards carbon monoxide and hydrocarbons.

The rates of conversion of a reducing catalyst for the individualhazardous components depend strongly on the exhaust gas temperature.With increasing exhaust gas temperature, the oxidation of hydrocarbonsand carbon monoxide is initiated first and oxidizing rates of more than70% are achieved within a temperature interval of about 150 to 175° C.As the temperature increases further the conversion of hydrocarbonsremains approximately constant. The exhaust gas temperature at which arate of conversion of 50% for the particular hazardous substance isachieved is called the light-off temperature for this hazardoussubstance.

The rate of conversion for nitrogen oxides varies in the same way as therate of conversion for hydrocarbons. However, it does not increaseregularly, but passes through a maximum at temperatures at which theoxidation of the hydrocarbons has virtually reached its maximum valueand then decreases with increasing temperatures to almost zero. Optimumrates of conversion for nitrogen oxides are therefore achieved only in anarrow temperature window. The hydrocarbons and carbon monoxidecontained in the exhaust gas are required as reducing agents.

The conversion curves for the individual hazardous substance dependstrongly on the formulation of the particular catalyst. This alsoapplies to nitrogen oxides. The position and width of the temperaturewindow and the maximum degree of conversion which can be achieved inthis window depend on the catalyst formulation. So-called lowtemperature catalysts have been disclosed and reach their maximumnitrogen oxide conversion at temperatures between 200 and 250° C. In thecase of high temperature catalysts, the maximum for nitrogen oxideconversion is situated above 300°C.

Reducing catalysts in the prior art have a maximum conversion fornitrogen oxides in oxygen-containing diesel exhaust gas of more than 55%at a temperature of about 200° C. The full width at half-maximum of thereaction curve for nitrogen oxides is about 100° C.

These types of catalysts are described, for example, in “Design Aspectsof Lean NO_(x) Catalysts for Gasoline and Diesel Engine Applications” byLeyrer et. al. in SAE No. 952495, 1995, and in “Catalytic reduction ofNO_(x) with hydrocarbons under lean diesel exhaust gas conditions” byEngler et. al. in SAE No. 930735, 1993. Catalysts based on zeoliteswhich may be exchanged with a variety of catalytically active metals(for example copper or platinum) are used. Further reducing catalystsare described in patent application DE 196 14 540.6 which is not a priorpublication.

The strong temperature dependence of the rates of conversion of nitrogenoxides represents a major problem during the purification of dieselexhaust gases because the engine outlet temperature of the exhaust gasesfrom diesel vehicles during operation can vary between about 100 and600° C. depending on the actual driving conditions. High rates ofconversion are therefore only achieved during brief phases of operationduring which the exhaust gas temperature is within the optimum range forthe catalyst used.

DE 40 32 085 A1 discloses a catalyst arrangement for reducing nitrogenoxides in an exhaust gas which is produced within a wide range ofexhaust gas temperatures. The arrangement consists of at least twocatalyst beds which are arranged directly one after the other andconsist of different catalyst materials which have their greatestcatalytic effect in different, adjacent zones of the exhaust gastemperature range. The catalyst bed in which the optimum effect isachieved at a higher temperature is arranged upstream of the othercatalyst bed.

Furthermore, DE 39 40 758 C2 discloses the treatment of exhaust gas fromdiesel engines by using oxidizing catalysts. For space reasons, thevolume of catalyst which is required is frequently divided between onecatalyst in the engine compartment and one catalyst in the under-floorregion. The use of reducing catalysts has also been disclosed. Toimprove the nitrogen oxide conversion, additional diesel fuel, as areducing agent, is frequently injected into the exhaust gas upstream ofthe catalyst.

DE 36 42 018 describes this type of arrangement. In order to remove thecarbon monoxide and excess hydrocarbon which is not consumed duringnitrogen oxide conversion by oxidation, the reducing catalyst may beconnected to a second catalyst which may be designed as a simpleoxidizing catalyst.

Hydrocarbons and carbon monoxide can be efficiently converted by knowncatalyst systems. The conversion of nitrogen oxides, however, is stillunsatisfactory. It reaches satisfactory values only during specificoperating phases of the engine.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a catalyst systemwhich has an optimum rate of conversion for all hazardous substances indiesel exhaust gas over a wide range of operating states for a dieselengine.

This object is achieved by a catalyst system for the treatment ofexhaust gases from a diesel engine consisting of a first and a secondcatalyst which are arranged in series in the exhaust gas treatment unit.The catalyst system is characterised in that both catalysts are reducingcatalysts and the first is located near the engine in a region of theexhaust gas treatment unit in which the exhaust gas temperature reachesmore than 200 to 300° C. when the engine is under full load and thesecond catalyst is located further from the engine, wherein the exhaustgas temperature in this region of the exhaust gas treatment unit whichis further from the engine is within the temperature window of thesecond catalyst, when the engine is under full load.

In this region of the exhaust gas treatment unit which is further fromthe engine, the exhaust gas temperature reaches a maximum of 500° C.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be apparent to those skilled in the art with reference tothe accompanying drawings, in which:

FIG. 1 is a cross-sectional view of an inlet face of a catalystaccording to an embodiment of the invention;

FIG. 2 is a cross-sectional view of an inlet face of a catalystaccording to another embodiment of the invention;

FIG. 3 is a cross-sectional view of an inlet face of a catalystaccording to a further embodiment of the invention;

FIG. 4 depicts an exhaust gas treatment unit according to the inventionassociated with a vehicle.

DETAILED DESCRIPTION OF THE INVENTION

In a preferred embodiment, the first catalyst has one or more zones withlower catalytic activity than the remainder of the catalyst, whichpermit slippage of non-converted hydrocarbons and carbon monoxide whenthe engine is running under full load.

The first catalyst is intended, according to the invention, to have thelowest possible light-off temperatures for the hazardous components. Incombination with being incorporated close to the engine it is thereforeable to convert the hazardous substances at a very early point duringthe cold-start phase. However, this means that the temperature windowfor the conversion of nitrogen oxides is passed through very rapidly. Atthe end of the cold-start phase hydrocarbons and carbon monoxide arevirtually fully converted by the first catalyst. The rate of conversionfor nitrogen oxides, however, drops to approximately zero due to thehigh exhaust gas temperatures of up to 600° C. achieved during operationunder full load.

In order to obtain high conversion of nitrogen oxides in this operatingstate, a second reducing catalyst, according to the invention, isincorporated further from the engine in the exhaust gas unit. Theposition of this second catalyst is in a region of the exhaust gas unitin which the exhaust gas temperature does not exceed a maximum of 500°C. even when the engine is running under full load. The invention usesthe known fact that the exhaust gas cools down while flowing through theexhaust gas pipe. Given an exhaust gas temperature of 600° C., atemperature loss of about 50 to 100° C. per meter of exhaust pipe can beexpected. According to the invention the second reducing catalyst islocated in a region of the exhaust gas unit in which the exhaust gastemperature is within the temperature window for nitrogen oxideconversion in the second catalyst when operating under full load andover long periods. To achieve this objective, it may be necessary toforce cool the exhaust gas by means of cooling fins welded onto theexhaust pipe. If, for structural reasons, a very long exhaust pipe isrequired between the two catalysts then, conversely, it may also berequired to provide the exhaust pipe with thermal insulation in order toprevent the exhaust gas cooling down too much.

The actual exhaust gas temperature which needs to be established at thesecond catalyst naturally depends on the catalyst formulation selectedfor the second catalyst and on the position of its temperature windowfor nitrogen oxide conversion. Accordingly, the previously-mentionedtemperature of at most 500° C. is understood to be only a guideline.

When the diesel engine operates continuously or under full load, carbonmonoxide and hydrocarbons are converted almost completely by the firstcatalyst, whereas the nitrogen oxides in this operating state pass overthe catalyst unchanged. However, the nitrogen oxides cannot easily beconverted even on the second reducing catalyst because the carbonmonoxide and hydrocarbons have already been converted on the firstcatalyst and there is now a lack of reducing agent in the exhaust gas.Therefore, in a preferred embodiment of the invention, the firstreducing catalyst is designed with zones of reduced catalytic activityas compared with the other zones so that even at high exhaust gastemperatures there is a certain degree of slippage of non-convertedhydrocarbons and carbon monoxide which then act as reducing agents forthe second reducing catalyst.

In the simplest case, these zones with reduced catalytic activity canconsist of an empty passageway in the first catalyst. Alternatively, itis possible to produce this zone with reduced catalytic activity as anannular zone at the periphery of the first catalyst. It is also possibleto produce segments or sectors as zones with reduced catalytic activity.The volume of these zones in comparison with the total volume of thefirst reducing catalyst must be selected in such a way that the slippageof non-converted hydrocarbons and carbon monoxide provides an amount ofreducing agent which is sufficient to convert the nitrogen oxides in thesecond reducing catalyst.

In a preferred embodiment of the invention, the zones with reducedcatalytic activity in the first reducing catalyst are provided with anadsorber for hydrocarbons. At the beginning of the cold-start phasehydrocarbons are adsorbed here and only desorbed after a time delay as aresult of the increasing exhaust gas temperature. At this point thesecond reducing catalyst connected in series is able to at least partlyconvert the desorbed hydrocarbons.

A further improvement may be produced if the zones with lower catalyticactivity are provided with a smaller proportion of catalytically activecomponents. This proportion may be adjusted in such a way that theconcentration of catalytically active components in these zones is 0 to20% of the concentration in the other zones of the catalyst.

Any catalyst formulations mentioned in the prior art may be used ascatalyst formulations. The first catalyst should have the lowestpossible light-off temperatures and thus also have a temperature windowfor nitrogen oxide conversion which lies at the lowest possibletemperature. Light-off temperatures and temperature windows in thesecond catalyst and the exhaust gas temperature within the secondcatalyst during continuous and full load operation must be mutuallyadjusted in such a way that an optimum conversion of nitrogen oxides isproduced during these operating phases. There is a variety of catalystformulations which offers a person skilled in the art this possibilityand the temperature of the exhaust gas, when it reaches the secondreducing catalyst, can be adjusted to lie within the optimum temperaturewindow for the particular catalyst formulation used, by means ofstructural features.

In order to observe these provisos, the second reducing catalyst isgenerally chosen to have a maximum for nitrogen oxide reduction at atemperature which is higher than the temperature for maximum nitrogenoxide reduction of the first catalyst.

The zones with reduced catalytic activity in the first reducingcatalyst, when using catalyst supports with a honeycomb structure forexample, may be obtained by blocking the flow channels in the intendedzones before coating the catalyst support with the catalytically activecoating. A material is selected as blockage material which can be burntaway without leaving a residue during calcination of the coating.Furthermore, it is possible to mask the zones intended to have reducedcatalytic activity before the coating procedure. If the slippage ofnon-converted hydrocarbons and carbon monoxide which is required isintended to take place through a hole in the catalyst structure, thiscan be taken into account during preparation of the substrate or thehole may be drilled out of the catalyst support prior to the coatingprocedure.

A variety of embodiments for the zones with reduced catalytic activityin the first reducing catalyst are shown in FIGS. 1 to 3. The figureseach show a view of the inlet face of a catalyst with a honeycombstructure, through which runs a set of parallel exhaust gas channels.The catalytically active zones of the catalyst, that is the internalwalls of the exhaust gas channels in these zones, are provided with acatalytically active coating, labelled with the number 1. The number 2denotes the zones in the catalyst with reduced catalytic activity. InFIG. 1 this zone consists of a central passageway. In FIG. 2 the zonewith reduced catalytic activity is designed as an annular zone at theperiphery of the catalyst structure. In this zone, the internal walls ofthe exhaust gas channels may be specified to be coating-free or may beprovided with an adsorber coating for hydrocarbons. However, this zonemay also have catalytically active components at a concentration of 0 to20% of the concentration of catalytically active components in zone 1.The catalytically active components in zone 2 may not necessarily beidentical to the catalytically active components in zone 1.

FIG. 3 shows the zone with reduced catalytic activity in the form of asegment. The precise shape of the zones with reduced catalytic activityis of no particular importance. The essential feature is only that thecross-section of these zones should be such that during continuous andfull load operation, sufficient slippage of non-converted hydrocarbonsand carbon monoxide takes place for reduction of nitrogen oxides to takeplace in the second reducing catalyst.

EXAMPLE

The structure of the catalyst system for a 2.2 liter direct injectiondiesel engine is described in this example.

1st Reducing Catalyst

A honeycomb structure made from cordierite with 62 cells, or flowchannels, per cm², a cell wall thickness of 0.2 mm, a diameter of 125 mmand a length of 180 mm, corresponding to a volume of 2.2 l, was providedwith a catalytic coating in accordance with examples 1, 9 and 17 frompatent application DE 196 14 540, apart from a 5 mm wide outer annularzone.

The reducing catalyst in this patent application is one in which acoating is applied to a catalyst support. The catalyst material inaccordance with DE 196 14 540 contains a mixture of a total of 5different zeolites with different moduli X (molar ratio SiO₂:Al₂O₃).Furthermore, the mixture contains an aluminium silicate activated withplatinum which has a concentration of silicon dioxide of 5 wt. % withrespect to the total weight of aluminium silicate. The specific surfacearea of this material in the fresh state is about 286 m²/g. The elementssilicon and aluminium are distributed very homogeneously through thematerial. Its crystal structure, in contrast to that of zeolites, isboehmitic. The preparation of an aluminium silicate of this type isdescribed in patent DE 38 39 580 C1.

The following procedure was used to prepare the first reducing catalyst,in accordance with DE 196 14 540:

First, the aluminium silicate was activated with platinum. To achievethis, it was placed in contact with an aqueous solution oftetraamineplatinum(II) hydroxide with constant stirring, so that a moistpowder was produced. After drying for 2 h at 120° C. in air, the powderobtained was calcined for 2 h at 300° C. in air. Then reduction wasperformed in a flow of forming gas (95 vol. % N₂ and 5 vol. % H₂) at500° C. for a period of 2 h. The Pt-aluminium silicate powder obtainedin this way contained 0.15 wt. % of platinum with respect to the totalweight.

An aqueous coating dispersion containing 40% solids was made up from thepreviously-prepared Pt-aluminium silicate powder. To this were added thefollowing zeolite powders in the ratio of 1:1:1:1:1: DAY (X=200);Na-ZSM5 (X>1000); H-ZSM5 (X=120); H-ZSM5 (X=40); H-mordenite (X=20).

The precise composition of the coating dispersion is given in table 1.

TABLE 1 Composition of the coating dispersion Raw material Composition(wt. %) Pt-aluminium silicate 67 H-mordenite (X = 20) 6.6 H-ZSM5 (X =40) 6.6 H-ZMS5 (X = 120) 6.6 DAY (X > 200) 6.6 Na-ZSM5 (X > 1000) 6.6

The initially-described honeycomb structure was coated by immersion inthe coating dispersion with an amount of 180 oxides (sic) per liter ofhoneycomb volume. The coating was dried in air at 120° C. and thencalcined for 2 h at 500° C. The coated honeycomb structure contained0.18 g of platinum per liter of honeycomb structure volume.

Then 30% of the length of this honeycomb structure was coated withanother catalyst material. Differently from the first catalyst material,the aluminium silicate in the second case was activated with 2.06 wt. %of platinum. Following the second coating procedure the honeycombstructure was dried and calcined in the same way as after the firstcoating procedure. The final catalyst contained 180 g of oxides perliter of catalyst volume with a platinum concentration of 0.18 g/l, as aresult of the first coating. The weight of oxides in the second coatingwas 39 g with an absolute amount of platinum of 0.8 g.

The catalyst prepared in this way contained different amounts of coatingalong the direction of flow of the exhaust gas. This producedtemperature windows for the reduction of nitrogen oxides which wereshifted with respect to each other along the catalyst. For the purposeof the invention, however, only the measurement of the hazardoussubstance conversion in the integrated temperature windows available inthe complete catalyst for the reduction of nitrogen oxides is critical.The variation in temperature windows along the catalyst is of lesssignificance to the invention. Therefore, within the context of theinvention, the expression ‘temperature window’ is always understood tomean only the temperature window for the entire catalyst.

The temperature window for the conversion of nitrogen oxides for thiscatalyst was between 180 and 250° C. with a maximum value for nitrogenoxide conversion under optimum conditions of 55% at 200° C.

2nd Reducing Catalyst

For the second reducing catalyst, a cordierite honeycomb structure ofthe same structural type as the first catalyst was used, but with avolume of only 1.8 l. This honeycomb structure was coated in the sameway as the first honeycomb structure, with the difference that thesecond coating was applied to two regions, each amounting to 15% of thetotal length of the honeycomb structure and starting from each of thetwo end faces of the honeycomb structure. Also, the entire honeycombstructure received a coating. The temperature window of this catalystwas between 220 and 300° C. with a maximum value for nitrogen oxideconversion under optimum conditions of 60% at 280° C.

Application Example

The two catalysts were incorporated into an exhaust gas treatment unitof a 2.2 l diesel vehicle in accordance with the diagram shown in FIG.4. The first reducing catalyst (4) was located just downstream of theengine outlet in a housing (5), the second reducing catalyst (6) waslocated in a housing (7) in the underfloor region (3) of the vehicle.

The nitrogen oxide conversions were measured with this exhaust gas unitduring the so-called new European driving cycle (MVEG-A driving cycle;EWG directive 70/220 EWG). This driving cycle comprises 4 urban drivingcycles at an average speed of 34 km/h and an open road cycle at amaximum speed of 120 km/h. The total duration of the MVEG-A cycle is1180 seconds.

Downstream of the catalyst located near to the engine, increasingexhaust gas temperatures between 80 and 180° C. were measured during the4 urban driving cycles. This temperature increased to values between 270and 300° C. during the open road cycle.

The average conversion for nitrogen oxides over the entire MVEG-A cyclewas 35%.

In another set of measurements, the reducing catalyst near to the enginewas replaced by a catalyst of the same size but which had the coatingdescribed for the first catalyst over its entire cross-section. Thiscatalyst thus had no annular zone with no coating. The averageconversion for nitrogen oxides measured with this system was 28%. It wasless than the value for the first set of measurements because, inparticular during the open road cycle with its high exhaust gastemperatures, the hydrocarbons could no longer pass through the firstcatalyst in the amounts required for optimum reduction of the nitrogenoxides on the second catalyst. Nevertheless, a satisfactory conversionof nitrogen oxides was produced over the MVEG-A cycle, even with thissystem.

The results of the two trials are summarised in the following table:

Catalyst system Nitrogen oxide conversion 1st reducing catalyst with 35%coating-free annular zone 1st reducing catalyst 28% without acoating-free annular zone

What is claimed is:
 1. A system for treating an exhaust gas stream froma diesel engine, the system comprising: a diesel engine which producesan exhaust gas stream; a first reducing catalyst, positioned along theexhaust gas stream at a first catalyst position, at which the exhaustgas temperature under full engine load is greater than 200° C., thefirst reducing catalyst being provided with at least one zone havinglower catalytic activity than the remainder of the catalyst, which zonepermits slippage of non-converted hydrocarbons and carbon monoxide underfull engine load; and a second reducing catalyst, positioned in serieswith the first reducing catalyst along the exhaust gas stream at asecond catalyst position downstream or the first catalyst position, atwhich the exhaust gas temperature is within a temperature window fornitrogen oxides conversion of the second reducing catalyst under fullengine load.
 2. The system according to claim 1, wherein the at leastone zone having lower catalytic activity comprises a passagewaycentrally located in the first reducing catalyst for the reduction ofnitrogen oxides.
 3. The system according to claim 1, wherein the atleast one zone having lower catalytic activity consists of an annularzone at the periphery of the first reducing catalyst for the reductionof nitrogen oxides.
 4. The system according to claim 3, wherein the atleast one zone having lower catalytic activity is provided with anadsorber for hydrocarbons.
 5. The system according to claim 1, whereinthe at least one zone having lower catalytic activity consists of asegment of the first reducing catalyst for the reduction of nitrogenoxides.
 6. The system according to claim 5, wherein the at least onezone having lower catalytic activity is provided with an adsorber forhydrocarbons.
 7. The system according to claim 1, wherein the at leastone zone having lower catalytic activity is provided with an adsorberfor hydrocarbons.
 8. The system according to claim 1, wherein the atleast one zone having lower catalytic activity contains from 0 to 20% ofan amount of catalytically active components which are present in aremainder of the first reducing catalyst for the reduction of nitrogenoxides.
 9. The system according to claim 1, wherein a maximum amount ofnitrogen oxide reduction in the first reducing catalyst occurs at atemperature which is lower than a temperature at which a maximum amountof nitrogen oxide reduction in the second reducing catalyst occurs. 10.The system according to claim 1, wherein the second catalyst position isa position at which an exhaust gas temperature under full engine load isno more than 500° C.
 11. The system according to claim 9, wherein thefirst reducing catalyst has a light-off temperature for conversion ofhazardous components which is lower than a light-off temperature forconversion of hazardous components of the second reducing catalyst. 12.The system according to claim 1, wherein an amount of slippage ofnon-converted hydrocarbons and carbon monoxide in the first reducingcatalyst under full engine load is sufficient to convert nitrogen oxidesin the second reducing catalyst.
 13. The system according to claim 2,wherein: a temperature window for conversion of nitrogen oxides in thefirst reducing catalyst is between 180° and 250° C., and a maximum valuefor the conversion of nitrogen oxides in the first reducing catalyst is55% and occurs at a temperature of 280° C.; and a temperature window forconversion of nitrogen oxides in the second reducing catalyst is between220° and 300° C., and a maximum value for the conversion of nitrogenoxides in the second reducing catalyst is 60% and occurs at atemperature of 280° C.
 14. The system according to claim 1, wherein thetemperature window for nitrogen oxides conversion of the second reducingcatalyst under full engine load is defined as a range of temperatures atwhich at least half of a maximum amount of reduction conversion occurs.15. The system according to claim 1, wherein: the first reducingcatalyst reaches a maximum nitrogen oxides conversion at a temperaturebetween 200 and 250° C.; and the second reducing catalyst reaches amaximum nitrogen oxides conversions at a temperature greater than 300°C.
 16. The system according to claim 1, wherein the first reducingcatalyst has a light-off temperature for conversion of nitrogen oxideswhich is lower than a light-off temperature for conversion of nitrogenoxides of the second reducing catalyst.